The standard for the Unified thread was agreed In November 1948 by the UK, the US and Canada to be used as the single standard for all countries using inch units. In 1965 the British Standards Institution issued a policy statement requesting that organisations should regard the BSW, BSF and BA threads as obsolescent. The first choice replacement for future designs was to be the ISO metric thread with the ISO inch (Unified) thread being the second choice.
The thread form diagrams below for Unified, ISO Metric Coarse and ISO Metric Fine threads are identical.
Metric threads are designated by the letter M followed by the nominal major diameter of the thread and the pitch in millimetres. For example M10 x 1.0 indicates that the major diameter of the thread is 10mm and the pitch is 1.0mm. The absence of a pitch value indicates that a coarse thread is specified. For example stating that a thread is M10 indicates a coarse thread series is specified of diameter 10mm (giving the thread a pitch of 1.5mm).
